qa/tasks: pass encoding to Key.get_content_as_string()

qa/tasks: pass encoding to Key.get_content_as_string()

we assume that boto.Key.get_content_as_string() returns str instead of
bytes, and compare the return value with a string, so, to ensure that
lhs and rhs are both strings, we need to decode the returned content.

since we always store strings composed with ASCII, it's safe to use
'ascii' to decode them.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 9ccf88d95fa5d184c56ac638bed37c3e3101356d)